U.S. patent number 6,861,975 [Application Number 10/603,847] was granted by the patent office on 2005-03-01 for chirp-based method and apparatus for performing distributed network phase calibration across phased array antenna.
This patent grant is currently assigned to Harris Corporation. Invention is credited to John Roger Coleman Jr., Travis Sean Mashburn.
United States Patent |
6,861,975 |
Coleman Jr. , et
al. |
March 1, 2005 |
Chirp-based method and apparatus for performing distributed network
phase calibration across phased array antenna
Abstract
A chirp-based arrangement derives a measure of phase variation
through a reference frequency transport cable of a phased array
antenna architecture, such as a spaceborne synthetic aperture radar
system. A direct digital synthesized chirp signal is injected in an
upstream direction into the transport cable from a downstream end
thereof, so that the chirp signal is transmitted in an upstream
direction, reflected from an upstream bandpass filter, and returned
in a downstream direction. At each of a plurality of nodes that are
distributed along the transport cable, the two chirp signals are
extracted and frequency domain-processed to derive said measure of
transport delay through the cable between the source of the
reference frequency signal and each of the nodes.
Inventors: |
Coleman Jr.; John Roger (Palm
Bay, FL), Mashburn; Travis Sean (Melbourne, FL) |
Assignee: |
Harris Corporation (Melbourne,
FL)
|
Family
ID: |
34193424 |
Appl.
No.: |
10/603,847 |
Filed: |
June 25, 2003 |
Current U.S.
Class: |
342/174; 342/118;
342/128; 342/131; 342/132; 342/165; 342/173; 342/195; 342/196;
342/25A; 342/25R; 342/352; 342/368; 342/371; 342/375 |
Current CPC
Class: |
H01Q
3/267 (20130101) |
Current International
Class: |
G01S
7/40 (20060101); G01S 007/40 () |
Field of
Search: |
;343/700MS,703,705-708
;342/25R-26D,118,128-133,165-175,192-197,368-384,352-356 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gregory; Bernarr E.
Attorney, Agent or Firm: Allen, Dyer, Doppelt, Milbrath
& Gilchrist, P.A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
The present invention relates to subject matter disclosed in our
co-pending U.S. patent application Ser. No. 10/603,843, filed Jun.
25, 2003, entitled: "Chirp-based Method and Apparatus for
Performing Phase Calibration Across Phased Array Antenna"
(hereinafter referred to as the '843 application), assigned to the
assignee of the present application, and the disclosure of which is
incorporated herein.
Claims
What is claimed is:
1. For use with an electrical apparatus having a signal transport
path along which a plurality of nodes are distributed, one of said
nodes being coupled with a reference frequency signal source that
is operative to generate a reference frequency signal that
propagates along said signal transport path and is distributed
thereby to others of said plurality of nodes, said signal transport
path imparting a variable delay therethrough of said reference
frequency signal employed by said apparatus to operate electrical
devices at said nodes, as a result of temperature variations along
said signal transport path, a method of providing a measure of
delay through said signal transport path between said one of said
nodes and a respective one of said others of said nodes, said
method comprising the steps of: (a) at a selected location along
said signal transport path that is farther away from said one of
said nodes than said others of said nodes, injecting a chirp signal
into said signal transport path so that said chirp signal
propagates as an upstream chirp signal over said signal transport
path and is reflected from said first node and propagates therefrom
along said signal transport path as a downstream chirp signal; and
(b) at said respective node, extracting and processing said
upstream chirp signal and said downstream chirp signal to derive a
measure of signal transport delay through said signal transport
path between said one of said nodes and said respective node.
2. The method according to claim 1, wherein said apparatus
comprises an antenna.
3. The method according to claim 1, wherein said apparatus
comprises a phased array antenna, and wherein said electrical
devices comprise array elements of said antenna.
4. The method according to claim 1, wherein step (b) comprises
mixing said upstream chirp signal and said downstream chirp signal
to derive a difference frequency signal, and subjecting said
difference frequency signal to a frequency domain operator to
derive said measure of signal transport delay through said signal
transport path between said one of said nodes and said respective
node.
5. The method according to claim 4, wherein said frequency domain
operator comprises a Fast Fourier Transform.
6. An apparatus comprising: a signal transport path along which a
plurality of nodes are distributed, said nodes having respective
electrical devices coupled therewith; a reference frequency signal
source coupled with one of said nodes and being operative to
generate a reference frequency signal that propagates along said
signal transport path and is distributed thereby to others of said
plurality of nodes, said signal transport path imparting a variable
delay therethrough, of said reference frequency signal employed by
said apparatus to operate said electrical devices at said nodes, as
a result of temperature variations along said signal transport
path; a chirp signal generator, coupled with said signal transport
path at a selected location therealong that is farther away from
said one of said nodes than said others of said nodes, and being
operative to inject a chirp signal into said signal transport path
so that said chirp signal propagates as an upstream chirp signal
over said signal transport path and is reflected from said first
node and propagates therefrom along said signal transport path as a
downstream chirp signal; and a chirp signal processor coupled with
a respective node of said others of said nodes, and being operative
to extract and process said upstream chirp signal and said
downstream chirp signal to derive a measure of signal transport
delay through said signal transport path between said one of said
nodes and said respective node.
7. The apparatus according to claim 6, wherein said apparatus
comprises an antenna.
8. The apparatus according to claim 6, wherein said apparatus
comprises a phased array antenna, and wherein said electrical
devices comprise array elements of said antenna.
9. The apparatus according to claim 6, wherein said chirp signal
processor is operative to mix said upstream chirp signal and said
downstream chirp signal to derive a difference frequency signal,
and to subject said difference frequency signal to a frequency
domain operator to derive said measure of signal transport delay
through said signal transport path between said one of said nodes
and said respective node.
10. The apparatus according to claim 9, wherein said frequency
domain operator comprises a Fast Fourier Transform.
11. The apparatus according to claim 6, wherein said one node has a
bandpass filter that is operative to pass said reference frequency
signal but reflect said chirp signal.
12. A method comprising the steps of: (a) distributing a plurality
of nodes along a signal transport path; (b) coupling a reference
frequency signal generator to one of said nodes, so that a
reference frequency signal generated thereby propagates in a
downstream direction along said signal transport path and is
distributed to others of said plurality of nodes, said signal
transport path imparting a variable delay therethrough of said
reference frequency signal, as a result of temperature variations
along said signal transport path; (c) at a selected location along
said signal transport path that is farther away from said one of
said nodes than said others of said nodes, injecting a chirp signal
into said signal transport path so that said chirp signal
propagates as an upstream chirp signal over said signal transport
path and is reflected from said first node and propagates therefrom
along said signal transport path as a downstream chirp signal; and
(d) at each of said other nodes, extracting and processing said
upstream chirp signal and said downstream chirp signal to derive a
measure of signal transport delay through said signal transport
path between said one of said nodes and said each node.
13. The method according to claim 12, wherein said apparatus
comprises an antenna.
14. The method according to claim 12, wherein said apparatus
comprises a phased array antenna, and wherein array elements of
said antenna are coupled to said nodes.
15. The method according to claim 12, wherein step (d) comprises
mixing said upstream chirp signal and said downstream chirp signal
to derive a difference frequency signal, and subjecting said
difference frequency signal to a frequency domain operator to
derive said measure of signal transport delay through said signal
transport path between said one of said nodes and said respective
node.
16. The method according to claim 15, wherein said frequency domain
operator comprises a Fast Fourier Transform.
Description
FIELD OF THE INVENTION
The present invention relates in general to communication systems
and subsystems therefor, and is particularly directed to a new and
improved, distributed chirp-based arrangement for deriving a very
accurate measure of phase variation through respective sections of
a reference frequency transport cable of a relatively physically
large phased array antenna architecture, such as a spaceborne
synthetic aperture radar system.
BACKGROUND OF THE INVENTION
Relatively large phased array antenna architectures, such as but
not limited to spaceborne, chirped synthetic aperture radar
systems, typically contain a multiplicity of transmitters and
receivers distributed across respective spaced apart arrays. In
such systems, a common, very precise reference frequency signal is
customarily supplied to both the transmit and receive array
portions. As such, there is the issue of how to take into account
phase shift associated with variations in the substantial length of
signal transport cable that links the reference frequency source,
which is customarily installed in one location of the array, with
the remaining portion of the array.
Because terrestrial open loop calibration of the system suffers
from the inability to take into account variation in temperature
along the transport cable due to changes in sun angle, and
variations in obscuration by components of the antenna support
platform in the antenna's space-deployed condition, it has been
proposed to perform temperature measurements at a number of
locations along the cable and provide phase compensation based upon
the measured values. A drawback of this approach stems from the
fact that there are non-linearities within the cable, so that over
different temperatures it is necessary to employ a larger number of
values in the calibration table. In addition, because this
technique performs multiple measurement points along the cable, it
introduces associated variations in loading which, in turn, produce
separate amounts of phase shift to the reference frequency
signal.
In accordance with the invention disclosed in the above-referenced
'843 application, this transport cable-based phase variation
problem is effectively obviated by injecting an RF chirp signal
into the signal cable from the remote end thereof, and correlating
the returned chirp that is reflected from the reference source end
with a delayed version of the injected chirp, to derive a measure
of the phase delay through the cable between its opposite ends.
Although this approach works quite well for a single length of
cable, it can become cumbersome when applied to a multinode system,
wherein the reference signal is to be delivered to a plurality of
spatially separated array sites. One straightforward approach would
be to implement a star-configured architecture, with each spoke of
the star containing its own dedicated chirp generator and
associated processing circuitry. Unfortunately, such an approach is
hardware intensive, and costly to implement.
SUMMARY OF THE INVENTION
In accordance with the present invention, this problem is
effectively obviated by employing a distributed network to connect
multiple array nodes with a single source of the reference
frequency signal, and injecting a single chirp from a far end node
of the distributed reference frequency transport medium toward the
reference frequency source node. The source of the reference
frequency signal is coupled to the reference frequency signal
transport medium by way of a bandpass filter, which is centered on
the output frequency of the reference frequency signal
generator.
A chirp signal, such as that produced by a direct digital
synthesizer, is injected onto the reference frequency signal
transport medium at a downstream-most end of the cable. The chirp
signal propagates `up` the cable in a `forward` direction and is
extracted at each of a plurality of sites or nodes to which the
reference frequency signal is distributed, before being reflected
from the bandpass filter and returning back `down` the cable in a
`reverse` direction.
Each reference frequency utilization location along the cable is
configured to extract the upstream-directed chirp signal and the
reflected and downstream-directed return chirp signal. These two
chirp signals are coupled to respective inputs of a mixer, the
difference frequency output of which is coupled to a frequency
domain operator, such as a Fast Fourier Transform (FFT)-based
operator. The FFT operator is operative to process the difference
frequency content of the output of the mixer to derive a measure of
the electrical distance between that respective site and the
reflective termination at the reference frequency signal source end
of the cable. Given this electrical distance the array signal
processor for that site determines the amount of phase shift which
the reference frequency undergoes in traversing the section of
cable between the reference frequency signal source end and the
site or node of interest.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 diagrammatically illustrates an embodiment of the
distributed node configured phase calibration architecture of the
present invention; and
FIG. 2 diagrammatically illustrates a non-limiting example of an
implementation of the FFT operator employed in the architecture of
FIG. 1.
DETAILED DESCRIPTION
Before describing in detail the distributed chirp-based phase
calibration arrangement of the present invention, it should be
observed that the invention resides primarily in a modular
arrangement of conventional communication circuits and components
and an attendant supervisory controller therefor, that controls the
operations of such circuits and components. In a practical
implementation that facilitates their being packaged in a
hardware-efficient equipment configuration, this modular
arrangement may be implemented by means of an application specific
integrated circuit (ASIC) chip set.
Consequently, the architecture of such arrangement of circuits and
components has been illustrated in the drawings by a readily
understandable block diagram, which shows only those specific
details that are pertinent to the present invention, so as not to
obscure the disclosure with details which will be readily apparent
to those skilled in the art having the benefit of the description
herein. Thus, the block diagram illustration is primarily intended
to show the major components of the invention in a convenient
functional grouping, whereby the present invention may be more
readily understood.
Attention is initially directed to the FIG. 1, wherein an
embodiment of the distributed chirp-based cable calibration
arrangement of the present invention is diagrammatically
illustrated. As shown therein, a reference frequency signal
generator 10, such as a very stable oscillator that drives a remote
antenna array 20, is coupled to a bandpass filter 30, which is
centered on the output frequency of the reference frequency signal
generator. Bandpass filter 30 is coupled to a first end 41 of a
length of cable 40, which serves to supply the reference frequency
signal produced by generator 10 to a plurality of remote array
sites 50-1, 50-2, . . . , 50-N distributed along the cable.
As pointed out above, one or more portions of the reference
frequency signal distribution cable 40 can be expected to be
subjected to temperature variations (and accompanying variations in
cable length/transport delay) due to changes in temperature, such
as those associated with changes in sun angle, and obscuration by
components of the antenna support platform. The present invention
solves this problem and provides an accurate measure of respective
sections of cable transport delay, by injecting a chirp signal from
a second or downstream-most end 42 of the cable. When so injected
by a chirp generator 60 (such as, but not limited to a direct
digital synthesizer (DDS)), the chirp signal propagates up the
cable in a `forward` direction and is extracted at each of the
distributed-sites 50-i, before being reflected from the bandpass
filter 30 and returning back down the cable in a `reverse`
direction.
Each location 50-i contains a pair of forward and reverse couplers
51 and 52, that are respectively operative to extract the
upstream-directed chirp signal shown at 45 in the frequency vs.
time diagram and the reflected and downstream-directed return chirp
signal shown at 46. The forward chirp signal processing path from
coupler 51 is coupled through an amplifier 61 to a first input 71
of a mixer 70. The reverse chirp signal processing path from
coupler 52 is coupled through amplifier 62 to a second input 72 of
mixer 70. The output of the mixer is coupled to a low pass filter
80, which is operative to couple the difference frequency output of
mixer 70 to a Fast Fourier Transform (FFT) operator 100.
FFT operator 100, shown in detail in FIG. 2 to be described, is
operative to process the difference frequency content of the output
of mixer 70 to derive a measure of the electrical distance between
site 50-i and the reflective termination (bandpass filter 30) at
the reference frequency signal source end 41 of the cable 40. Given
this electrical distance the array signal processor 90 for site
50-i may readily determine the amount of phase shift which the
reference frequency undergoes in traversing the section of cable
between reference frequency signal source end 41 and the site or
node of interest.
Referring now to FIG. 2, a non-limiting example of an
implementation of the FFT operator 100 is shown as comprising an
analog-to-digital (A/D) converter 110 that is coupled to sample the
difference frequency output of the low pass filter 80. The sampled
difference frequency data is subjected to an FFT 120, so as to
provide a relatively coarse measurement of the electrical distance
between the reference frequency signal source termination 41 and
the node of interest. The output of FFT 120 is then subjected to a
centroid finder 130, which reduces the relatively coarse electrical
distance measurement to a relatively fine electrical distance
value. The electrical distance value produced by centroid finder
130 is then converted into a phase offset value for that node's
cable delay by means of a unit converter 140.
It should be noted that the rate of change of cable length is
considerably slower relative to the processing time associated with
the operation of the invention. As noted previously, in an
environment, such as a spaceborne application, changes in cable
length due to temperature are ambient effects, such as sun angle
and obscuration by components of the antenna support platform. Such
changes are very slow relative to the high signal transport and
processing speeds associated with the generation of the chirp and
correlation processing of the chirp return, which may be in the
pico to microsecond range.
While we have shown and described an embodiment in accordance with
the present invention, it is to be understood that the same is not
limited thereto but is susceptible to numerous changes and
modifications as known to a person skilled in the art. We therefore
do not wish to be limited to the details shown and described
herein, but intend to cover all such changes and modifications as
are obvious to one of ordinary skill in the art.
* * * * *